PHOTOCHEMISTRY OF 7-ACETOXYBENZOPYRAN DERIVATIVES. SYNTHESIS OF EUPATORIOCHROMENE AND ENCECALIN

Article abstract of DOI:10.1016/S0040-4020(01)89221-8

The photolysis of the 7-acetoxybenzopyran derivatives 1a-5a has been carried out.Chromen 1a was found to undergo extensive photopolymerization.Chromanone 2a underwent a rather inefficient photo-Fries rearrangement to the 6- and 8- acetyl derivatives 2b and 2c.Diacetoxychromene 3a gave the unsaturated ketone 8 as the main product, besides its deacetylation analogue 9 and chromanone 11.Diacetoxychroman 4a afforded a mixture of four C-acetyl products: 1b, 1c, 2b and 2c, together with chromene 1a and chromanone 2a.Finally, irradiation of chroman 5a gave rise to a mixture of the two possible photo-Fries products 5b (43percent) and 5c (52percent).The mechanistic implications of the above results are discussed, with special emphasis on the photoreactivity of the phenyl ester as compared with that of the pyran ring, the enol ester and the benzyl ester moieties.The synthetic applications of these transformations are illustrated with the preparation of eupatoriochromene 1b and encecalin 1e.